The present invention relates to improvements in electrophotographic image forming methods and it relates more particularly to an improved method for forming an image of high contrast and free of fog by the simultaneous charge transfer process while permitting the repeated use of the electrostatic charge receiving dielectric member.
A simultaneous charge transfer method is described in U.S. Pat. No. 2,825,814 issued Mar. 4, 1958 and employs a photosensitive member including a photoconductive layer on a light transparent electrode plate (normally a NESA treated glass plate) and an electrostatic charge receiving dielectric member in the form of a flexible belt including a layer of a few microns thickness of a highly insulative dielectric material formed on a flexible electrode. The surface of the photoconductive layer of the photosensitive member is firmly held in face-to-face contact with the surface of the dielectric layer of the belt and then a direct current voltage of 500 to 1000 volts is applied between the light transparent electrode plate of the photosensitive member and the flexible electrode of the dielectric belt simultaneously with the exposure of the back of photosensitive member to a light image so as to form an electrostatic latent image of the light image on the surface of dielectric layer. Further, the use instead of the dielectric belt of electrostatic transfer paper in which a dielectric layer of high resistivity is coated onto an electroconductive layer of high resistivity is described in U.S. Pat. No. 3,502,408 issued Mar. 24, 1970.
Among the advantages or features of the aforesaid simultaneous charge transfer process are that a positive latent image can be formed from a negative original, that an electrostatic latent image can be formed in a short period of time without requiring a large number of steps, and that a high voltage source in the order of a couple of thousand volts, such as for corona discharge device, is not required. On the other hand, there is the disadvantage that the transfer efficiency at an air gap of less than 5.mu. or over 40.mu. between the photoconductive layer and the charge receiving dielectric layer markedly deteriorates and as a consequence when the normal techniques are utilized to effect the face-to-face contact between the photosensitive member and the dielectric member there results in heavy blurs in the image density of the final image. To avoid this, the value of the voltage applied may be increased so that the photosensitivity rises to reduce blurs in image density. However, such expedient causes the non-illuminated areas (i.e., portions on the surface of the dielectric member corresponding to non-exposed areas of the photosensitive member which in turn corresponds to dark portions of the original) to become charged thereby fogging the final copy.
There are various methods proposed to solve the above drawbacks. A first method is to maintain a uniform air gap between the photosensitive member and dielectric member by positioning number of plastic balls of few microns in diameter therebetween in a scattered arrangement in the manner described in U.S. Pat. No. 2,825,814. A second method is to apply a biasing voltage to the developing electrode at the time of development so as to lower the fog density of the image as described in Japanese Laid Open Patent Application No. 51-122450. A third method is to precharge the dielectric member prior to the image forming step, as described in U.S. Pat. No. 2,937,943 issued May 24, 1960, by applying a voltage of a polarity opposite to that of the voltage to be applied at the time of exposure to a light image simultaneously with the full illumination to light, and a fourth method as described in Japanese Patent Publication No. SHO 51-29019 is to apply a voltage of opposite polarity under conditions of darkness after the formation of the latent image so as to reduce the fogging of the image.
However, each of the above methods possesses disadvantages. In the first method the photoconductive layer as well as the dielectric layer are subject to damage by the plastic balls and handling of these plastic balls are difficult and inconvenient; in the second method, a mechanism is required for applying a biasing voltage to the developing electrode and such electrode is easily soiled; in the third method, the intensity of the illumination of the photosensitive member must be highly uniform in order to uniformly charge the dielectric member and the applied voltage, the intensity of the illumination and the amount of time the voltage is applied must be accurately controlled and maintained in order to always charge the dielectric member to a constant surface potential; and in the fourth method, fogging is not completely prevented but still remains to an undesirable degree and additionally, the step of applying the voltage in the dark cannot be conducted until there is absolutely no influence from the light used to expose the original.
Aside from these disadvantages, where the electrostatic charge receiving dielectric member is to be repeatedly used with a latent image being formed on the dielectric member by the simultaneous charge transfer process and this image being then developed and subsequently transferred onto a copy paper, there is the need after the cleaning of the residual toner to erase any residual charge from the dielectric member in order that the same may be repeatedly used. Conventional methods for the removal of residual charges involve the use of an A.C. corona discharger as described in U.S. Pat. No. 2,777,957 or the use of a metal roller carrying a biasing voltage in which the roller is brought into contact with the surface on which the residual charges are present as described in Japanese Laid Open Patent Application No. SHO 49-53044. However, each of these methods require special devices and the control of their operation is highly complicated.
Additionally, if there is employed an image forming method wherein the dielectric member is precharged to a polarity opposite to the polarity of the latent image and a voltage is then applied simultaneously with the exposure to the light image, there will be charges of both positive and negative polarities remaining on the dielectric member and the erasure thereof is more complicated than the case of only single polarity.